Virtual tape libraries (VTL) transform disk drives into virtual tape to improve backup/restore times, as well as to provide other benefits. A VTL operates by providing a layer or interface between a host or client computer and a disk drive system. This VTL layer acts like one or more tape drives with one or more tapes or pieces of media and mimics commands to and from the client computer to make the client believe that it is communicating with tape drives. Instead, however, the VTL actually exchanges data with one or more disk drives.
A Jan. 3, 2007 article by B. Pariseau in ComputerWeekly.com, entitled, “Storage Outlook '07: In Search of Better Data Management,” indicates that some consumers prefer tape and VTL because drives capable of up to 4 to 1 compression save storage resources. According to the article,                The VTL improves upon some of the traditional limitations with physical tape libraries. With VTLs [one] can create hundreds of virtual tape drives. Each server can have its own dedicated drives, reducing the complexity of shared resources. The SCSI tape protocol has some downsides shared by both VTL and physical tape . . . . Neither enables concurrent reads from the same volume by two separate hosts. This means tape replication for [data recovery] and restores cannot happen at the same time.        
Tape by its nature is sequential and thus does not permit simultaneous read operations from different locations on a given tape. Disk drives, however, do permit simultaneous read and write commands. But, as noted above, a VTL forces the disk drive to operate like a tape drive and thus does not permit concurrent reads, or other capabilities incompatible with tape. Many limitations of VTLs exist because of a lack of disk management solutions or limitations in backup software applications that employ VTLs. For example, when a write command is provided to a drive, the VTL reserves that drive and no other commands can be performed.
The need exists for a system that overcomes the above problems, as well as providing additional benefits. Overall, the examples herein describe some prior or related systems, and their associated limitations are intended to be illustrative and not exclusive. Other limitations related to existing or prior systems will become apparent to those of skill in the art upon reading the following Detailed Description.
In the drawings, the same reference numbers and acronyms identify elements or acts with the same or similar functionality for ease of understanding and convenience. To easily identify the discussion of any particular element or act, the most significant digit or digits in a reference number generally refers to the Figure number in which that element is first introduced (e.g., element 310 is first introduced and discussed with respect to FIG. 3).